Prostate cancer is the most common cancer, excluding skin cancers, in American men. The American Cancer Society estimates that during 2002 about 189,000 new cases of prostate cancer will be diagnosed in the United States. Accurate determination of the extent of local disease in the prostate is difficult. Current imaging techniques include, for example, transrectal ultrasound (TRUS), endorectal coil magnetic resonance imaging (MRI), and proton magnetic resonance spectroscopic imaging (MRSI). The reported accuracy of TRUS for determining if prostate cancer is confined within the capsule varies widely from 58% to 90%. However, preliminary data from recent studies of endorectal MRI show higher accuracy (75-90%) than TRUS, and better consistency.
In addition to morphologic extent, directed biopsy and assessment of tumor aggressiveness are important for accurate staging and treatment for prostate cancer when there is an elevated PSA. Current biopsy techniques are based on random spatial sampling and have a lower than desired sensitivity (60-70%) for identification of carcinoma of the prostate. Early preliminary studies of combined MRI/MRSI demonstrated localization of cancer to a sextant of the prostate with sensitivity up to 95% and specificity up to 91%. However, more specifically localized biopsies, rather than randomly taken biopsies, would be desirable.
MRI is presently regarded as the best imaging modality for assessing soft-tissue tumors like prostate cancer. This is confirmed by numerous reports in the literature. In an early study, carried out from December 1987 to April 1989, Rifkin et al [7] report on the collaborative effort of five institutions that are part of the Radiological Diagnostic Oncology Group. More than 200 patients who were thought clinically to have localized cancer of the prostate were studied preoperatively with both MRI and transrectal ultrasonography to evaluate the ability of these techniques to determine the exterit (stage) of the tumor. They underwent radical prostatectomy, and radiologic and pathological findings were correlated. The overall staging accuracy of ultrasonography was 58% (126 of 219 patients), with a standard error of 3%. The overall staging accuracy of MRI was 69% (133 of 194 patients), with a standard error of 3%. The subject invention can increase the diagnostic accuracy of MRI when combining MRI scans with interventional biopsy techniques.
Prostate cancer is the second most common cause of cancer death in US men. Its incidence is on the rise because more cancers are detected due to wide-ranging screening programs using either digital rectal exams or serum prostate-specific antigen (PSA). Whenever abnormalities crop up in these examinations, the patient is traditionally referred for ultrasound-guided biopsy, which has a low sensitivity and a specificity of only 60% for cancer detection [3]. This is why ultrasound is often used just to guide biopsies. However, MRI performs much better at cancer detection.
Typical prostate biopsies are performed by palpation (whether or not a nodule is present) or using ultrasound guidance (when a visible lesion is present). However, endorectal ultrasound is not sensitive enough for a screening tool. The visibility of the anterior capsule is poor as is visualization of seminal vesicle and lymph node involvement. Extracapsular disease and lymph node involvement is better picked up with MR, although interobserver variability is quite high (positive predictive value ˜70%). PSA and proton MR spectroscopy get higher ratings for predicting the Gleason grade. Patients with incompatible PSA and biopsy results or MR spectroscopy results or with MR visible lesions would thus benefit from an MR guided prostate biopsy.